This invention relates to a gas blast circuit breaker and more particularly to a gas blast circuit breaker provided with an insulation nozzle disposed in the vicinity of an arc generating section so as to blast extinguishing gas, such as SF.sub.6 gas, to an arc generated between a movable contact and a stationary contact when large electric current is interrupted.
Recently, with an increasing consumption of electric power, electric devices have been required to operate under high voltage and large electric current. In a gas blast circuit breaker, which is a final protective device for an electric power system, it is necessary to provide an insulation nozzle capable of withstanding high voltages.
To meet this requirement, a new nozzle construction has been proposed differing from a conventional nozzle with the new nozzle construction being achieved by advanced techniques of analysis such as a gas flow analysis.
In such a nozzle construction disclosed, for example, in Japanese Patent Unexamined Publication No. 60-218722 corresponding U.S. Pat. No. 4,667,072, a high-pressure gas region space is formed at a downstream side of a throat section of the nozzle by a normally-slanting surface which extends along the direction of flow of an extinguishing gas and a reversely-slanting surface intersecting this normally-slanting surface, and a region near a distal end portion of a stationary contact constitutes the high-pressure gas region until the stationary contact passes through this space position, thereby making it possible to enhance voltage performance.
One method of enhancing the internal arc resistance of the nozzle has been proposed, for example, in Japanese Patent Unexamined Publication No. 57-210507, in which 20% by volume of boron nitride (BN) is mixed as a filler in a fluoroplastic material of the nozzle.
With respect to the nozzle disclosed in the above-mentioned Japanese Publication 60-218722, it has been experimentally determined, as described in the specification thereof, that, the shape of the reversely-slanting surface and the diameter of the throat section greatly influence the dielectric interrupting performance.
On the other hand, in this type of nozzle, in order to enhance the internal arc resistance, it is necessary that boron nitride should be mixed in the nozzle material, as disclosed in the above-mentioned Japanese Publication 57-210507. In this case, however, it is not considered how much the energy lines of the arc intrude into the nozzle, and there exists a portion on the surface of the nozzle where the absorption of the arc energy is increased. This results in a drawback that the surface consumption by the arc is increased, and the above-mentioned nozzle construction suffers from the problems that the shape and size of the reversely-slanting surface are changed by the consumption with the surface result being that the intended performance can not be achieved after large electric current is interrupted many times.